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Flat underbody in FD

Old 10-19-07, 04:38 PM
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Flat underbody in FD

Back around '02 there was a lot of talk about flat underbodies (from what i could find in the forum searching) but I couldnt find any threads where anyone acutally put it on their car. So 5 years later, any updates?

I'm thinking of making one, just a simple bolt on (probably in multipul secions to allow access to anyhting underneath the car without removing the whole underbody) thats as close to being level and on the same plain (no wierd channels and hopefully no undulations) as possible until connecting to the rear diffuser. The idea behind this just to clean up airfow underneath the car to minimize turbulence - not to take advantage aero principle to increase downforce by channeling the air like the IRL cars and old f1 cars (id hate to hit a bump and loose the "suction" i was depending on for cornering and hit the wall).

If anyone has any experience with this or is able to offer any tips that would be excellent.

thanks in advance
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Old 10-19-07, 04:41 PM
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I think most are made of fiberglass or CF, but I'm not quite sure.

Also, anyone who makes a single piece underbody panel is a retard when they realize they have to change the oil.
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Old 10-19-07, 05:01 PM
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Originally Posted by Xeonith View Post
Also, anyone who makes a single piece underbody panel is a retard when they realize they have to change the oil.
It would be hallarious to watch though!

... any ideas on minium materal thickness? Doing a quick google search i found some 4mm 12in x 12in pannels for $45 or some 4.7mm 12in x 12in pannels for $237.
http://k-mac-plastics.net/carbon-fiber-sheets.htm
and
http://www.machv.com/cafiplsh.html

... i suppose it would also be wise to ask which website and companies are best to deal with when purchasing CF sheeting or fiberglass sheeting.
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Old 10-19-07, 06:08 PM
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Much of the air that passes through the radiator exits under the car. Keep that in mind. I heard a story of a guy who did a flat floor on a Miata and had the car overheat in 3 laps or something rediculous because there was very little air getting through the radiator.
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Old 10-19-07, 11:39 PM
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Originally Posted by Black91n/a View Post
Much of the air that passes through the radiator exits under the car. Keep that in mind. I heard a story of a guy who did a flat floor on a Miata and had the car overheat in 3 laps or something rediculous because there was very little air getting through the radiator.

yeah thats a good point.. ive been trying to work out an idea on how to fix that. So far im thinking of a v-mount setup with a vented hood and then getting some carbon fiber or aluminium tubing to duct out radiator towards the sides of the car. If it would be wise i could also duct everything out the top through the hood but im not sure how fesible that would be.

One of the guys in the forums placed his intercooler on the bottom of the v-mount so it might be easyer to get proper ducting that way since there isnt any fans taking up valuable space between the cooling system and the bottom of the chassis.
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Old 10-20-07, 07:56 AM
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the biggest problem a flat under body poses is lift. you cant do a completely flat panel you need to have grooves to channel the air to defusers that will counter the lift created by the compression of the air between the car and the ground, a nose splitter will help that too. Lexus actually uses a floor pan in the new LS that is a bunch of divots much like an inverted golf ball under the new LS sedans to reduce lift.

Ive thought on this subject a lot over the years and the biggest problem is controling the air over the suspension as it moves up and down.
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Old 10-20-07, 12:34 PM
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From pics I've seen, even factory flat underbody cars aren't entirely flat.
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Old 10-20-07, 01:35 PM
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http://www.fatcatmotorsports.com/iga...o_articles.zip

These are a bunch of aero articles from Racecar Engineering that used to be hosted on the Advantage CFD site but got taken down when they went to an exclusive relationship with Honda F1. Luckily I'd saved them on my HD and now these guys are hosting the files that I'd saved for everyone's benefit. It includes articles on airdams, splitters, flat floors, underbody downforce and so on. Read it and you will be much further ahead. Many of the (race)cars that use flat floors have ducts that let the air out from the back of the fenders and probably all have ducted hoods.
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Old 10-21-07, 12:35 AM
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Oh wow, thanks Black91n/a ... thats a great amount of information!

I think ill try a few different designs and see what works better emperically... if it does make any noticeable difference in track time and feel.

a little off topic but:

if anyone wants some aero info I have a few good sites too, im almost tempted to save each page on an offline html file.
http://www.ae.su.oz.au/aero/aerodyn.html

and...

http://www.lerc.nasa.gov/WWW/K-12/airplane/bga.html

the last site is nice and interactive... its not car specific but its not as air can tell the difference between a plane or a car... it also explains low speed low altitude aero effects as well as (and probably more importantly) why "normal" explinations of aero principles are wrong and the deficencies of them.
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Old 10-22-07, 06:44 PM
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I run a splitter/diffuser up front with a wing out back to balance it out. Works quite well. I think the only way to get a real flat underbody is to build a tube frame and design it as a key element. With low side skirts and a rear diffuser I think you could generate a great deal of low drag downforce.

-Trent
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Old 10-24-07, 01:42 AM
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Originally Posted by TrentO View Post
I run a splitter/diffuser up front with a wing out back to balance it out. Works quite well. I think the only way to get a real flat underbody is to build a tube frame and design it as a key element. With low side skirts and a rear diffuser I think you could generate a great deal of low drag downforce.

-Trent
lets see some pics. sharing is caring.
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Old 12-02-08, 02:02 AM
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Back from the dead-

Originally Posted by Black91n/a View Post
http://www.fatcatmotorsports.com/iga...o_articles.zip

These are a bunch of aero articles from Racecar Engineering that used to be hosted on the Advantage CFD site but got taken down when they went to an exclusive relationship with Honda F1. Luckily I'd saved them on my HD and now these guys are hosting the files that I'd saved for everyone's benefit. It includes articles on airdams, splitters, flat floors, underbody downforce and so on. Read it and you will be much further ahead. Many of the (race)cars that use flat floors have ducts that let the air out from the back of the fenders and probably all have ducted hoods.
I just wanted to say that link was the most useful amount of Areo info I have ever read-and directly explained why my Colt got light and spun out at 200+MPH. Seriously, anyone who races anything in any arena involving automobiles should read this, most importantly the last one about splitters and air damns.


Great info.

Last edited by coltboostin; 12-02-08 at 02:05 AM.
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Old 12-02-08, 09:26 AM
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Here area few pics from ALMS. These cars aren't tube frame.



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Old 12-02-08, 10:01 AM
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Also when thinking about designs for this dont forget about your exhaust heat. If you box your exhaust in over the floorpan you might see a huge amount of extra heat dissipating into the cabin.
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Old 12-02-08, 09:50 PM
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Those are about as close as you can get to a tube frame car without actually having it be a "tube frame" car. Not much of the body is left at all, well, at least the Corvettes are like that anyway. They may not be allowed as much leeway in GT2 and not everyone will necessarily go to those lengths.

Not only will the exhaust cook the cabin, it'll cook itself, cook the diff and the tranny. You will almost certainly need to add coolers to those too.

There is definitely benefits to flat floors, but it's only part of the puzzle, for instance you need to be really low to make them work very well, which means very stiff suspensions to keep from bottoming and very stiff in roll too to keep it flat to prevent loss of downforce on the corners.
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Old 12-02-08, 11:36 PM
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Also it's quite important to have it quite stiff in pitch motions, ie dive under braking and squat on acceleration (to control the angle of the underfloor, it can be quite sensitive).
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Old 12-17-08, 09:29 AM
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heres a pic of a street chassis 240sx with a flat bottom. Tube frame isn't necessary. He used alumalite which is two sheets of aluminum with a corrugated core. Much cheaper than CF.

Plus, a flat bottom would reduce lift on the car just because its smoother and allows air to flow faster. Higher velocity equals lower pressure. This effect outweighs any pressure gain from air compression and there would be a reduction in lift/increase in downforce overall before adding any diffusers or venturi tunnels.

The naca ducts on the back are for the differential cooling, I think he ran two lines connecting the transmission and the diff with two inline oil pumps moving fluid between them and a cooler on one line to cool both.
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Old 12-17-08, 09:43 AM
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nice project! is this someone you know? It would be interesting to hear the drivers feedback of the car with the underbody and without. Also I would love to hear about any change in laptimes to back up the theoretical info.

The alumacore or alumalite stuff is great, but it still has quite a bit of weight to it so he is deff piling on some ballast there with those big sheets.
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Old 12-17-08, 10:13 AM
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http://www.ziptied.com/forums/index.php?topic=7879.0 Its Sasha Anis' car and heres a long thread following the development and end to this car. If you wanna get to the good stuff, go backwards thru the thread.
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Old 12-17-08, 12:24 PM
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I work on Ferrari's, Maserati's and Aston Martins all day... Ferrari's and Astons both have flat panels running the length of the car from factory.

Aston Martin uses a pretty thin set of underbody panels all made of aluminum.

Ferrari takes a different approach using extremely thin fiberglass for the underpanel... (think around 1mm possibly).]

Both vehicles are not tube frame, nor do they have perfectly flat underpanels... the majority of the underpanels stay on the same "plane" underneath the vehicle, however they have MANY vents underneath and a few NACA ducts normally... I am still at work, so I can't elaborate much more right now. However, if it would benefit anyone I could try to take a few pictures of some vehicles to show examples...

- Chris
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Old 12-17-08, 03:44 PM
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If I remember from the articles, the best would be to try for a slight rake from the front of the car to the rear. For material I'd use thin aluminum sheet with longitudinal beads rolled for strength. Fasten it up using dzus fasteners to hold it in place. The whole concept hinges on your ability to keep air out from under the car so you can nreate a low pressure area. You'll need pretty low side skirts and a low splitter and diffuser. The reason I'd suggest tube frame is you can design it in as opposed to fitting it after the fact. Here'a quick workup for a tube frame FD I've been thinking on. The plan would be to build a rake into the bottom of the chassis the entire length of the car. The splitter and diffuser would be removable elements which are ground up designed into the car.

-Trent
Attached Thumbnails Flat underbody in FD-3rdgen4.jpg   Flat underbody in FD-3rdgen3.jpg   Flat underbody in FD-3rdgen5.jpg  
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Old 07-26-13, 03:43 PM
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Its hard to find right material that is cheap, firm, and light.
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Old 08-08-13, 08:07 AM
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I have a full underbody flat floor with front splitter, middle section and rear diffuser. I will get some pictures of it shortly for you.

It is made from Carbon, as the grp would be too weak and shatter whenever it was hit on a curb...either that or ridiculously heavy as it would need to be very thick.

Only problem with the diffuser is not being able to have an OE fuel tank in to get the correct angle. So we went for a bag tank in where the bins would be behind the front seats
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Old 08-11-13, 09:31 AM
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Originally Posted by MinotaurRacing View Post
I have a full underbody flat floor with front splitter, middle section and rear diffuser. I will get some pictures of it shortly for you . . .
Do you have any issues with heat buildup? The two cars I know well that have been modified with flat floors are an AC Cobra and a Viper. Both of those have side pipes so they avoid trapping exhaust heat. BTW - They're both aluminum, affixed with Dzus fasteners.
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